Global warming, linked to the greenhouse gases released by fossil fuels, has incited to develop alternative energy solutions, which do not emit such gases during their operation, such as, for example, solar modules. A solar module comprises a “photovoltaic cell”, this cell being capable of converting light energy into electricity. Photovoltaic (PV) cells are encased in an “encapsulant”, and an upper protective layer and a lower protective layer are positioned on both sides of the encapsulated cell. The encapsulant must perfectly take up the shape of the space existing between the photovoltaic cell and the protective layers in order to avoid the presence of air, which would limit the output of the solar module. The encapsulant must also prevent the contact of the cells with water and oxygen from the air, in order to limit the corrosion thereof. Moreover, the encapsulant must offer optimal and durable transparency to the solar radiation and a good adhesion with the protective layers inside the cell, and this during the entire life of a solar module, which has to be at least of 20 years. As PV cells become thinner, the stress generated by encapsulant, especially at low temperatures, needs to be low. The modulus of elasticity of said encapsulant, needs thus to be as low as possible.
The most prevalent solution is the use of encapsulant compositions based on ethylene/vinyl acetate copolymer (EVA) as disclosed in JP 1019780. EVA has good transparency properties. However, its adhesion to the protective layers is not satisfactory and coupling agents have to be added to the encapsulant composition, generally chosen between organic silane or titanate. Moreover, EVA decomposes under solar radiation and at high temperature, resulting in acetic acid release that corrodes the inside of the photovoltaic cell. EVA-based encapsulant compositions tend to turn yellow in time, thus lowering the yield of the solar module. Also, EVA has poor adhesion to transparent plastics, such as polymethyl methacrylate (PMMA), used in concentrating photovoltaic (CPV) devices.
It has already been generally suggested (PCT/IL 2009/001064) to use light curable liquid encapsulant formulations comprising: at least one acrylic polymer (defined as a polymer having at least 50% of its chains made of repeating units derived from acrylic and/or methacrylic acid, ester or amide thereof); at least one unsaturated monomer and/or oligomer and at least one photoinitiator. This encapsulant is capable of bonding to a transparent amorphous surface, and thus it is suitable to encapsulate a photovoltaic cell, especially for CPV applications.
Commercially available acrylic (or methacrylic) polymers such as Elvacite™, manufactured by Lucite, provide the encapsulant with reasonable adhesion to plastics, especially PMMA, but not excellent. When these polymers have a glass transition temperature (Tg) greater than 0° C., large amount of plasticizer are required to lower the Tg and the modulus of elasticity. The plasticizer is subjected to migration from compound during service, and also may phase-separate at sub-zero temperatures, causing undesired haze. Usually, plasticizers, even of aliphatic chain type, are more subjected to thermal degradation than acrylate and methacrylate polymers. Moreover, said polymers have limited compatibility with unsaturated monomers and thus tend to haze during photo-curing.
There is thus a need to develop new encapsulant materials for photovoltaic applications, with improved properties especially in terms of adherence, transparency, low glass transition temperature and durability.